How real is the multiverse?

Imagine setting off in a rocket and leaving Earth. Leaving the solar system. Leaving our galaxy. Breaking through the edge of the observable universe and leaving our cosmos behind (which would be impossible, as you'd have to go faster than the speed of light, but work with me here).

Now you are cruising through the unfathomable void for eons, only to come upon another universe, with another galaxy inside it, with another solar system, another Earth … and another you, sitting there, reading this article.

This is the multiverse, and it might be a natural prediction of the physical theories that define the beginning of the universe. Or it might not. It's tough to say, as new research has shown.

Related: What is multiverse theory?

A big old universe

Cosmologists largely believe that when our universe was extremely young — less than a trillionth of a trillionth of a second old — it went absolutely nuts. In the tiniest fraction of a moment of time (again, involving trillionths upon trillionths of a second), the universe got really, really big.

How big? It's hard to say exactly, because this concept is highly hypothetical, but "way bigger than you might think" should suffice. Most models of this event, called inflation, call for a universe that is at least 10^52 times bigger than the observable volume of the cosmos. Since that observable patch is already 90 billion light-years across, that means that the true extent of our universe is so big, it's nearly incomprehensible.

Inflation solves a lot of problems in standard Big Bang cosmology — a model that describes how the universe began — like the fact that regions of the universe vastly distant from each other have roughly the same temperature. According to inflation theory, those regions were once much cozier and got to know each other pretty well, before inflation ripped them apart.

There's another potential consequence of inflation: It may not be done. Indeed, it may never be done. This is called "eternal inflation," and this idea describes how the universe at the grandest scales may always be inflating, with only tiny pockets pinching off to become normal, sedate patches like our own. Each pinched-off island universe would be separated by a vast gulf of nothingness, with the islands flying away from each other faster than light (because that's what inflation does).

These island universes, embedded within the larger "multiverse," would never meet and could never talk to each other. In fact, it would be impossible to find direct evidence of their existence. 

To inflate or not to inflate

Without that direct evidence, could we at least make an educated guess as to whether the multiverse is likely or not? If we're just one bubble in a giant bathtub filled with foam expanding faster than light, how could we figure this out?

The first step is to test inflation. The jury is still out on that, but there is some evidence that something like inflation happened in the early universe. The fluctuations in the cosmic microwave background, or the light released when our universe began to cool when it was 380,000 years old, have a pattern that matches what you'd see if inflation had occurred. No other theory of the early universe matches that pattern of light.

So that's good. But "inflation" isn't a single theory. It's more like a class or category of theories. Different models assume different physics, different drivers, different causes and different effects of this event. As all of these theories are based on hypothetical models of the extreme physics of the early universe, it's too early to tell which of the theories — if any — are correct.

Physicists suspect that eternal inflation is generic, meaning a  consequence of most, if not all, models of inflation. So, following this suspicion, if inflation is correct, then eternal inflation is also likely correct, and the multiverse might be real.

Judging the multiverse

Needless to say, the existence of the multiverse is a pretty big pill to swallow. If eternal inflation is correct, then there isn't just one universe, or a lot of universes, but an infinite number of pocket universes. Each one would potentially support its own laws of physics and arrangements of particles. So if the number of ways to arrange matter and energy is finite — there are only so many ways you can construct a universe — then an infinite multiverse demands repeated copies of the same physical situation, even if any particular combination of physical configurations is incredibly rare.

That means there's a copy of you, at some finite (but very far) distance away. And another copy past that. And another. And another. An infinity of you's doing this exact same thing.

But we can only say the multiverse is likely if eternal inflation is indeed generic (that is, a common feature of most, if not all, inflation models), which is exactly what a team of physicists claims in a recent paper, published to the preprint database arXiv and submitted to the Journal of Cosmology and Astroparticle Physics. They put a large number of inflation models through a grinder, varying the types of models and the model parameters, counting which ones were a one-and-done affair and which ones led to eternal inflation and a multiverse.

Their answer: It's complicated.

First off, they found that eternal inflation wasn't nearly as common as originally thought. Their explanation for why cosmologists had thought eternal inflation was generic was because those earlier cosmologists had studied only a limited set of models. They found that many viable inflation models ("viable" here means they didn't obviously contradict observations) didn't lead to an eternally inflating scenario.

However, the researchers found that it's tough to even get a handle on measuring the "commonality" of something like eternal inflation, since we don't have a good grasp of inflation models and how they work. They argued that it's impossible to answer the question of genericness with a single answer, because there's so much we have yet to learn about the physics of inflation.

So is there another you out there, reading this exact same article? Science says: It's tough to say.

Originally published on Live Science.